In recent years, research has been vigorously conducted on an organic thin-film light emitting device that emits light upon recombination of an electron injected from a cathode and a hole injected from an anode in an organic light emitting body interposed between both the electrodes. The light emitting device has been attracting attention because of the following features. The device is thin and emits light having high luminance under a low driving voltage, and the selection of its light emitting material allows the device to emit light beams of various colors.
When a voltage is applied to an organic electroluminescence device (hereinafter referred to as “organic EL device”), a hole and an electron are injected into a light emitting layer from an anode and a cathode, respectively. Then, the hole and the electron thus injected recombine in the light emitting layer to form an exciton. At this time, singlet excitons and triplet excitons are produced at a ratio of 25%:75% according to the statistical law of electron spins. When the organic EL devices are classified in accordance with their light emission principles, the internal quantum efficiency of a fluorescence-type organic EL device is said to be at most 25% because the device uses light emission based on a singlet exciton. On the other hand, it has been known that as a phosphorescence-type organic EL device uses light emission based on a triplet exciton, its internal quantum efficiency is enhanced to 100% when intersystem crossing from a singlet exciton is efficiently performed.
Optimum device design has been conventionally performed in the organic EL devices depending on their fluorescence- and phosphorescence-type light emission mechanisms. In particular, it has been known that when a fluorescent device technology is simply diverted for the phosphorescence-type organic EL device, owing to its light emitting characteristic, a high-performance device is not obtained. The reason for the foregoing is generally considered to be as described below.
First, phosphorescent emission is light emission utilizing a triplet exciton and hence a compound to be used in the light emitting layer must have a large energy gap. This is because a value for the energy gap of a certain compound (hereinafter, sometimes referred to as “singlet energy”) is typically larger than a value for the triplet energy of the compound (which refers to an energy difference between its lowest excited triplet state and ground state in the present invention).
Therefore, in order that the triplet energy of a phosphorescent emitting dopant material may be efficiently trapped in the device, first, a host material having a larger triplet energy than the triplet energy of the phosphorescent emitting dopant material must be used in the light emitting layer. Further, an electron transporting layer and a hole transporting layer adjacent to the light emitting layer must be provided, and a compound having a larger triplet energy than that of the phosphorescent emitting dopant material must be used in each of the electron transporting layer and the hole transporting layer. Designing an organic EL device on the basis of a conventional device design idea as described above leads to a situation where a compound having a larger energy gap than that of a compound to be used in the fluorescence-type organic EL device is used in the phosphorescence-type organic EL device. As a result, the driving voltage of the entire organic EL device increases.
In addition, a hydrocarbon-based compound having high oxidation resistance or high reduction resistance that has been useful in a fluorescent device has a small energy gap because of large expansion of its π-electron cloud. Accordingly, such hydrocarbon-based compound is hardly selected in the phosphorescence-type organic EL device, and an organic compound containing a heteroatom such as oxygen or nitrogen is selected. As a result, the phosphorescence-type organic EL device involves a problem in that its lifetime is short as compared with that of the fluorescence-type organic EL device.
Further, device performance is largely affected by the fact that the exciton relaxation rate of a triplet exciton of the phosphorescent emitting dopant material is extremely long as compared with that of a singlet exciton. That is, light emission from a singlet exciton has a fast relaxation rate leading to the light emission, and hence the diffusion of the exciton into a peripheral layer of the light emitting layer (such as the hole transporting layer or the electron transporting layer) hardly occurs and efficient light emission is expected. On the other hand, light emission from a triplet exciton is spin-forbidden and has a slow relaxation rate. Accordingly, the exciton is apt to diffuse into the peripheral layer, and thermal energy deactivation occurs from a compound except a specific phosphorescent emitting compound. In other words, the control of a region where an electron and a hole recombine is more important in such device than in the fluorescence-type organic EL device.
By such reason as described above, an improvement in the performance of the phosphorescence-type organic EL device requires material selection and device design different from those in the case of the fluorescence-type organic EL device.
One of the most serious problems in the organic thin-film light emitting device is compatibility between high current efficiency and a low driving voltage. A method involving doping a host material with several percent of a dopant material to form a light emitting layer has been known as means for obtaining a high-efficiency light emitting device (see Patent Literature 1). The host material is requested to have a high carrier mobility, uniform film formability, and the like, and the dopant material is requested to have a high fluorescent quantum yield, uniform dispersibility, and the like.
Although a fluorescent (singlet light emission) material has been conventionally used as the dopant material in general, an attempt has been made to use a phosphorescent (triplet light emission) material for enhancing current efficiency since the past, and a group of Princeton University has shown that the material provides much higher current efficiency than the conventional fluorescent material does (See Non Patent Literature 1). There has been disclosed a technology involving using, as the phosphorescent dopant material, a metal complex containing iridium, osmium, rhodium, palladium, platinum, or the like as a central metal (see Patent Literatures 2 to 4). In addition, there has been disclosed a technology involving using, for example, a carbazole derivative, an aromatic amine derivative, or a quinolinol metal complex as the host material to be combined with the phosphorescent dopant material (see Patent Literatures 2 to 6). However, none of the materials has shown sufficient current efficiency and a low driving voltage.
Meanwhile, a technology involving using a biscarbazole derivative as a hole transporting material for a fluorescent device has been disclosed (Patent Literature 7). Some technologies each involving using a biscarbazole derivative as a phosphorescent host material have also been disclosed. For example, Patent Literature 8 describes an example of a biscarbazole derivative as a host material to be combined with a specific metal complex dopant. However, no biscarbazole derivative compound that causes the expression of a high light emitting characteristic has been disclosed. In addition, Patent Literature 9 describes that a biscarbazole derivative is used as a host material. In Patent Literature 9 described above, a substituent for improving the carrier transportability of the host material such as an amino substituent-containing phenyl group, a naphthyl group, or a fluorenyl group is introduced into the N-position of a carbazole structure. Although a reduction in the driving voltage of a light emitting device has been achieved by the introduction, a specific effect of the introduction on the lifetime of the device has been unclear.
Meanwhile, several technologies each involving extracting light emission from a triplet exciton, which have not been effectively exploited so far, have been disclosed in relation to a technology for enhancing the efficiency of a fluorescence-type device. For example, Non Patent Literature 2 discloses the following mechanism by analyzing a non-doped device using an anthracene-based compound as a host. Two triplet excitons collide and fuse with each other to produce a singlet exciton, and as a result, the intensity of fluorescent emission is increased. The phenomenon in which the two triplet excitons collide and fuse with each other to produce a singlet exciton as described above is hereinafter called a triplet-triplet fusion (TTF) phenomenon.
In addition, Non Patent Literature 3 discloses a blue light emission fluorescence-type OLED including a layer formed of an aromatic compound (efficiency-enhancement layer referred to as “EEL”) between a light emitting layer containing a host and a dopant, and an electron transporting layer. It has been shown that an OLED using a compound EEL-1 in its EEL is driven at a low voltage, shows high external quantum efficiency, and has a long lifetime as compared with an OLED using BPhen or BCP in its EEL. It can be said that the EEL functions as a barrier layer for causing the TTF phenomenon.
Further, an organic EL device using an EEL that causes the TTF phenomenon requires a hole transporting layer for adjusting a carrier balance.